Genetic Influences Affecting Alcohol Use Among Asians.

Aldehyde dehydrogenase (ALDH) is one of the two enzymes primarily involved in alcohol metabolism. Several variants exist of the gene that produces ALDH. One of these gene variants, which generates a nonfunctional enzyme, is present in Asians but not in Caucasians and African-Americans. People with two copies of the defective gene respond to alcohol consumption with intense flushing and other unpleasant reactions, such as nausea. Consequently, these people consume very little alcohol and are at a much lower risk for alcoholism than people with functional ALDH genes. People with one copy of the defective gene also flush after ingesting alcohol and are at relatively lower risk for alcoholism than people with fully functional genes. In addition, these people have more intense, but not necessarily less pleasant, reactions to alcohol as assessed by both physiological and psychological measures. People with the defective gene variant also respond to alcohol consumption with characteristic changes in brain activity.

A lcoholism 1 is a complex disorder vulnerability for alcoholism, and provides moderate amount of alcohol (Chan 1986). arising from both biological (i.e., a theoretical perspective of the results. This reaction, although documented in genetic) and sociocultural (i.e., Chinese poetry as early as the first century environmental) factors. During the past two B.C., first was reported in the scientific lit decades, studies in humans assessing indi ALCOHOLINDUCED erature in 1972 (Wolff 1972). The flushing vidual differences in the response to alcohol FLUSHING AMONG ASIANS reaction is characterized by a rapidly in have increased our understanding of how creased blood flow to the skin of the face, genetic traits may contribute to a person's People of Asian descent consistently expe neck, and chest; other symptoms may in rience lower levels of alcoholism and higher predisposition to alcoholism (Schuckit 1994). clude increased heart rate (i.e., tachycardia), rates of abstinence than other ethnic groups Some studies have aimed at identifying decreased blood pressure (i.e., hypotension), (Helzer et al. 1990;Klatsky et al. 1983). factors that might increase the risk for the headache, nausea, and vomiting (Chan 1986). Researchers have offered two explanations disorder, whereas others have addressed The manifestations of the flushing response for these observations (reviewed in Sue and protective factors. Most studies have focused vary widely: Some people intensely experi Nakamura 1984). An "environmental" hypo on people who statistically are at high risk thesis suggests that cultural values among for alcoholism, such as children of alcoholic Asians emphasize drinking in moderation.
Tamara L. Wall, Ph.D., is an assistant professor fathers, although some research has exam Conversely, the "genetic" theory proposes in the Department of Psychiatry at the Univer ined people with a statistically lower risk, that Asians experience a different physio sity of California, San Diego, California, and an such as those of Asian heritage. This article logical reaction to alcohol. Bridging this adjunct member in the Department of Neuro presents evidence that both alcohol use and dichotomy, Sue and Nakamura (1984)  ence the full range of symptoms, whereas others have significantly milder reactions after consuming the same alcohol dose.

THE GENETIC BASIS FOR THE FLUSHING REACTION
The alcoholinduced flushing response is associated with the process by which the body metabolizes alcohol. Two enzymes are primarily involved in alcohol metabolismalcohol dehydrogenase (ADH), which con verts alcohol to acetaldehyde, and aldehyde dehydrogenase (ALDH), which converts acetaldehyde to acetate (figure 1). Re searchers hypothesize that an elevated level of acetaldehyde, a highly reactive and poten tially toxic byproduct, in the blood and tissue causes the flushing reaction (Harada et al. 1981). Two mechanisms could con tribute to increased acetaldehyde levels: a higherthannormal acetaldehyde production by ADH or a slowerthannormal acetalde hyde breakdown by ALDH.

Different Forms of ADH and ALDH
Both ADH and ALDH exist in different forms (i.e., isoenzymes) in the body. Iso enzymes are groups of enzymes that per form the same chemical reaction but have a slightly different amino acid composition and different kinetic properties. 2 ALDH, for example, has four isoenzymes: three that primarily exist in the cell's cytoplasm and one-called ALDH2-that is located in the cell's energyproducing structures, the mitochondria. ALDH2 is responsible for most of the acetaldehyde breakdown in the cell. Thus, alcoholinduced flushing and other symptoms related to alcohol sensi tivity have been attributed primarily to an ALDH2 deficiency (Harada et al. 1981).
Each ALDH molecule consists of four parts, or subunits, all of which are produced (i.e., encoded) by the same gene. The gene encoding the ALDH2 isoenzyme is called ALDH2. This gene exists in two variants, or alleles. One allele, ALDH2 1 , encodes a functional enzyme subunit; the other allele, ALDH2 2 , contains a small change in the gene and thus encodes a defective subunit that causes ALDH deficiency. Because every person inherits two copies of each gene-one from the mother and one from the father-three different combinations of ALDH2 alleles (i.e., three different ALDH2 genotypes) are possible (figure 2): A person can have two ALDH2 1 alleles (ALDH2 1 /2 1 genotype), one ALDH2 1 allele and one Figure 1 The pathway of alcohol metabolism. Once in the liver, alcohol is converted into acetaldehyde and the acetaldehyde is converted into acetate. The enzyme alcohol dehydrogenase (ADH) catalyzes the first half of alcohol metabolism, and the enzyme aldehyde dehydrogenase (ALDH) catalyzes the second half. NAD + is a coenzyme that plays an accessory role in enzyme catalysis.
NAD + Alcohol Acetaldehyde Acetate ADH ALDH NAD + ALDH2 2 allele (ALDH2 1 /2 2 genotype), or two ALDH2 2 alleles (ALDH2 2 /2 2 genotype). A person with two identical alleles (e.g., ALDH2 1 /2 1 genotype) is called homozy gous for that allele; a person with two different alleles is called heterozygous. The ALDH2 genotype determines how well the ALDH2 enzyme functions: An enzyme containing any defective subunits encoded by the ALDH2 2 allele is less active than an enzyme containing only functional subunits encoded by the ALDH2 1 allele. Fur thermore, an enzyme consisting only of defective subunits (i.e., in a person homozy gous for ALDH2 2 ) will be less active than an enzyme consisting of both functional and defective subunits (i.e., in a heterozygous person). As a result of the reduced enzyme activity in people with at least one ALDH2 2 allele, the conversion of acetaldehyde to ace tate is slowed, creating excess levels of blood acetaldehyde after alcohol consumption.

Racial Differences in the Frequency of ALDH2 Alleles
Researchers can determine a person's ALDH2 genotype using DNA extracted from a small blood sample. Such molecular analyses have found that the distribution of ALDH2 alleles varies among different ethnic groups (Goedde et al. 1992): Nearly all people of Caucasian and AfricanAmerican descent are homozygous for the functional ALDH2 1 allele. Among Asians, however, only about 50 percent are homozygous for ALDH2 1 , 30 to 40 percent are heterozygous, and 5 to 10 percent are homozygous for the defective ALDH2 2 allele.
As described previously, these geno types among Asians predict their response to alcohol. People who have at least one ALDH2 2 allele experience elevated ac etaldehyde levels as well as a readily ob servable facial flush following alcohol ingestion. Asians homozygous for ALDH2 1 , however, generally have no, or only a mild, flushing response. 3

ALDH2 Genotype and Flushing as a Protective Factor for Alcoholism
The physiological characteristics of flush ing are similar to the reactions to alcohol in people taking the medication disulfiram (i.e., Antabuse™), which is used to deter alcoholics from drinking. Molecular stud ies found that disulfiram mimics ALDH2 deficiency by inhibiting the enzyme and thus causing increased levels of acetalde hyde after alcohol consumption (Harada et al. 1982). The similarity between the effects of disulfiram and the flushing reaction suggests that the aversive effects of ac etaldehyde may discourage drinking or excessive drinking in Asians and thus serve as a protective factor against alcoholism.
Several studies demonstrating signifi cantly lower levels of alcohol use and alco holism in Asians with ALDH2 2 alleles support this idea. For example, people homozygous for the ALDH2 2 allele drink very little alcohol (Takeshita et al. 1994) and are not found among alcoholics (Thomasson et al. 1991;Chao et al. 1994). Asians who are heterozygous drink significantly less and are much less likely to be alcoholic than Asians homozygous for the functional ALDH2 1 allele 4 (Chao et al. 1994;Takeshita et al. 1994;Thomasson et al. 1991). The heterozygous genotype does not provide full protection from alcoholism, however, because 12 to 19 percent of Chinese and Japanese alcoholics have one ALDH2 2 allele (Chao et al. 1994;Thomasson et al. 1991).
Thus, a growing body of evidence suggests that possession of at least one ALDH2 2 allele reduces a person's alcohol use and predisposition to alcoholism. The findings demonstrate that a genetic factor can influence, but not necessarily prede termine, the development of alcoholism. Each person's predisposition to alcoholism probably depends on both genetic and envi ronmental factors. Studies of alcoholics who are heterozygous for the ALDH2 gene may allow researchers to learn more about the interactions among genes or between genes and environmental factors that affect the development of alcoholism.

Physiological and Psychological Consequences of ALDH Genotype
Recent studies have investigated in more detail the physiological response to alcohol in Asians having different ALDH2 geno types (Wall et al. 1992(Wall et al. , 1993(Wall et al. , 1994Wall and Ehlers 1995). In these studies, the sub jects were male, Americanborn university students of Chinese, Japanese, and Korean descent whose ALDH2 genotypes were de termined from blood samples. Each subject was tested on two occasions after receiving either a placebo or 0.75 milliliters of alcohol per kilogram of body weight (mL/kg, equal to two to three alcoholic drinks). All subjects reacted comparably to the placebo beverage and had similar blood alcohol concentra tions (BAC's) after consuming the alcoholic beverage. Alcohol, however, induced very different physiological reactions in subjects, corresponding to the three different geno types described earlier. For example, two subjects who were homozygous for the de fective ALDH2 2 allele became ill after drink ing the alcoholic beverage, experiencing nausea, vomiting, extreme tachycardia, and hypotension. These observations confirm the assumption that people who are homozy gous for ALDH2 2 are most likely to experi ence severe physiological reactions; their intolerance of alcohol may help to explain why no one with this genotype has been found to be alcoholic.
The ALDH2 genotype appears to affect not only physiological reactions but also more subjective responses to alcohol. In the experiment described above, the subjects were asked to assess the effects the alco holic beverage had on them. The men who were heterozygous for ALDH2 reported more intense, but not necessarily more un pleasant, reactions to alcohol than the men with two functional ALDH2 1 alleles, even though they had similar BAC's and similar recent drinking patterns. For example, heterozygous subjects reported both signif icantly stronger "effects of alcohol" than subjects homozygous for ALDH2 1 (figure 3) and more intense attributes of intoxication, such as feeling "high," "great overall," and "drunk." The heterozygous subjects also tended to more strongly experience the negative effects of intoxication, such as feeling "uncomfortable," "nauseated," and "terrible overall," but statistically these re sults were not significantly different from the responses of the subjects homozygous for ALDH2 1 .
These findings contradict the hypothesis that people who are heterozygous for ALDH2 drink less because they experience only aversive alcohol effects. In fact, the

Frequency Band Alcohol Dose
Low Moderate High (< 0.5 g/kg) (0.5-1.0 g/kg) (> 1.0 g/kg) (less than 2-3 drinks) (3-6 drinks) (more than 3-6 drinks) delta (< 4 Hz) theta (4.0-7.5 Hz) increased increased slow alpha (7.5-9.0 Hz) decreased increased fast alpha (9.0-12.0 Hz) decreased decreased beta (> 12.0 Hz) increased 1 The brain waves measured with an electroencephalogram (EEG) are grouped into several "bands," each containing waves with a certain frequency range. The frequency indicates how often a wave recurs in a certain time and is measured in hertz (Hz). 1 Hz = 1 cycle per second. SOURCES: Summarized from Begleiter and Platz 1972;Ehlers et al. 1989;and Kalant and Woo 1981. results suggest that heterozygous people who regularly consume alcohol may have a more intense positive response to alco hol than those homozygous for ALDH2 1 . Thus, one mechanism that may contribute to the decreased rate of alcohol use and al coholism among Asians with at least one ALDH2 2 allele could be heightened sensitiv ity to relatively low doses of alcohol (Wall et al. 1992). A person with this heightened sensitivity would need to drink less to ex perience the same response to alcohol as a person with lower sensitivity. Consequently, a person carrying the ALDH2 2 allele may avoid high intake of alcoholic beverages and thus be less likely to develop alcoholism. If the ALDH2 genotype affects not only people's physiological responses but also their subjective feelings of intoxication, possession of an ALDH2 2 allele also may influence a person's vulnerability to alco hol at the level of brain functioning. The following sections describe research aimed at understanding how different combina tions of ALDH2 alleles may affect the brain's response to alcohol.

MEASURING THE BRAIN'S RESPONSE TO ALCOHOL
Electrophysiological techniques provide noninvasive ways to assess various ele ments of brain function. One of the most common methods, the electroencephalo gram (EEG), is performed by attaching electrodes to a person's scalp and recording the spontaneous electrical brain activity, known as brain waves. Brain waves can be divided into several different groups-slow alpha, fast alpha, beta, delta, and thetaaccording to their frequency 5 (see table 1). Changes in brain waves (e.g., their ampli tude 6 ) correlate with changes in the level of consciousness or different psychological states. EEG's allow researchers to charac terize and quantify the brain's spontaneous electrical activity and provide information about brain responses to external sensory stimuli. These responses also are called eventrelated potentials (ERP's).
To measure ERP's, the subjects are exposed to stimuli (e.g., sounds or lights) while their EEG is being recorded. When an uncommon stimulus occurs (e.g., a red light in a sequence of green and yellow lights), the brain produces a characteristic pattern of brain waves. One ERP wave that has been studied frequently in alcohol re search is called P300 because it can be detected about 300 milliseconds after the uncommon stimulus. Although scientists believe that the P300 wave reflects specific neurocognitive functions (e.g., the brain's evaluation of the stimulus and the process of selecting a response), they do not know the physiological basis for these functions.
Twin and family studies indicate that many EEG parameters are largely geneti cally determined (van Beijsterveldt and Boomsma 1994). A smaller number of studies also suggest moderate to high heritability for ERP's (van Beijsterveldt and Boomsma 1994). Both EEG parame ters and ERP's are sensitive to the effects of acute and chronic alcohol use (Porjesz and Begleiter 1985).
Alcohol's acute effects on EEG pat terns have been studied since the 1930's. In general, low doses of alcohol that result in mild intoxication and behavioral activa tion (less than 0.5 gram of alcohol per kilo gram of body weight [g/kg], corresponding to fewer than two to three drinks) reduce the amplitudes of slow and fastalpha waves and increase the amplitudes of the beta waves (table 1). Moderate alcohol doses (0.5 to 1.0 g/kg) usually result in increased slowalpha and theta waves and decreased fastalpha waves. Larger doses of alcohol (more than 1.0 g/kg), which are associated with sedation and drowsiness, produce increases in the amplitude of the theta waves (Begleiter and Platz 1972;Ehlers et al. 1989;Kalant and Woo 1981). The magnitude of these alcohol effects, how ever, varies significantly among people.
The genetic vulnerability for alco holism, as inferred from a family history of the disorder, partly determines a person's brain response to alcohol. For example, children of alcoholics, who are at elevated risk for developing alcoholism, have a different EEG response to alcohol from control subjects without alcoholic relatives (Cohen et al. 1993;Ehlers and Schuckit 1991). Sons of alcoholic fathers in particu lar may have a less intense EEG response to alcohol, possibly reflecting an innate lower level of sensitivity to alcohol (Cohen et al. 1993;Ehlers and Schuckit 1991).
Not only does alcohol consumption affect the spontaneous brain activity re flected by the EEG patterns, it also alters brain responses to external stimuli (i.e., the ERP's). Acute alcohol administration reduces the amplitude of P300 and/or increases the time between the stimulus and the appearance of P300 (i.e., the latency), suggesting reduced efficiency in brain processing (Porjesz and Begleiter 1985). The intensity of alcohol's effect on P300 varies according to the dose ingested (Rohrbaugh et al. 1987). A person's genetic vulnerability for alcoholism also appears to affect the P300 response. For example, alcoholinduced P300 changes were smaller in men at high risk for developing alco holism (i.e., men having an alcoholic father) than in men at lower risk (Elmasian et al. 1982;Schuckit et al. 1988).

ALDH2 Genotype and the Electrophysiological Response to Alcohol
Because electrophysiological responses appear to be correlated with a person's genetic predisposition to alcoholism, re searchers have used EEG patterns and ERP's to evaluate alcohol's effects on brain functioning in college students of Asian origin with different ALDH2 genotypes (Wall et al. 1993;Wall and Ehlers 1995).

Figure 4
Mean event-related potentials in 15 subjects homozygous* for ALDH2 1 and 14 heterozygous* subjects after drinking a placebo beverage (solid line) or a beverage containing 0.75 ml/kg alcohol (dotted line). The subjects had to press a button when they heard a rarely presented tone. In both groups, alcohol delayed the appearance of the P300 wave (the segment occurring between 300 and 450 msec) and decreased P300 amplitude, but the effects were stronger in the heterozygous subjects than in the homozygous subjects. The subjects received either a placebo or a moderate dose of 0.75 mL/kg alcohol before their brain waves were recorded. Subjects homozygous for the functional ALDH2 1 allele showed a typical EEG response, in cluding increased theta and slowalpha and decreased fastalpha activity.
In the heterozygous subjects with one ALDH2 2 allele, however, the slowalpha activity decreased significantly compared with that of the subjects homozygous for ALDH2 1 . This pattern of alcoholrelated EEG changes in the heterozygous subjects is consistent with the increased activation that may be caused by elevated acetaldehyde levels (Wall et al. 1993) and differs from the pattern seen in Caucasian sons of alcoholics (Ehlers and Schuckit 1991). These findings suggest that EEG responses may help to measure the effects of acute intoxication on brain functioning and genetically influenced reactions to alcohol.
The response to alcohol in the same subjects also was assessed using ERP's (Wall and Ehlers 1995). The study found that compared with a placebo, alcohol consumption in all subjects significantly decreased the amplitude and increased the latency of the P300 wave. In the heterozy gous subjects, however, alcohol's effects on P300 were significantly greater than in the subjects homozygous for ALDH2 1 , although all subjects had equivalent BAC's (figure 4). These data indicate that people heterozy gous for ALDH2 experience a more intense response to alcohol and that their brain func tioning may be more affected by alcohol than that of people homozygous for the ALDH2 1 allele.

THEORETICAL AND RESEARCH IMPLICATIONS
One model of genetic influences in alco holism suggests that a person's genetically determined reaction to alcohol may affect the likelihood of both drinking alcohol and developing alcoholism (Schuckit 1994). This theory predicts that compared with the general population, people at higher risk for developing alcoholism (e.g., children of al coholic fathers) may respond less intensely to alcohol, as assessed by measures such as subjective feelings of intoxication and elec trophysiological parameters. Conversely, people at lower risk for alcoholism (e.g., members of ethnic groups with a low preva lence of the disorder) would experience a heightened response to alcohol.
Studies reviewed in this article that examined the acute responses to alcohol in Asian men support this model. The men fell into three groups with respect to their genetic risk for developing alcoholism: extremely low risk (homozygous for the defective ALDH2 2 allele), moderately low risk (het erozygous for ALDH2), and relatively high risk (homozygous for the functional ALDH2 1 allele). The same groups were found to be highly sensitive, intermediately sensitive, and minimally sensitive to the effects of alcohol.
Although genetic factors may contribute to a person's risk for or protection from alco holism, their interactions with environmental factors determine their overall vulnerability. For example, as described here, the ALDH2 genotype helps determine the level of alco hol consumption among Asians. The pro posed mechanism, however, by which this gene influences the development of alco holism (i.e., by evoking a more intense response to alcohol in brain functioning) can only manifest itself after alcohol use and may even require a certain level of alcohol consumption. In contrast, environmental factors, such as cultural attitudes that em phasize abstinence or moderate alcohol consumption, can protect against the devel opment of alcoholism in the absence of al cohol use, when genetic factors, such as the ALDH2 gene, likely have no influence.
No other gene has been identified that influences the level of alcohol consumption and risk for alcoholism to the same extent as ALDH2. Therefore, further studies of Asians with known ALDH2 genotypes would provide a unique opportunity to improve our understanding of how the in teractions between genetic and environ mental factors determine a person's risk for alcoholism. Researchers have demonstrated, for example, that alcohol use among Asian Americans varies with the number of gen erations their families have lived in the United States and their level of accultura tion 7 (Chu et al. unpublished master's thesis 1978; Sue et al. 1979). Also, substantial differences in alcohol consumption patterns and the prevalence of alcoholism exist among different ethnic groups (e.g., Chinese, Japanese, and Koreans) (Helzer et al. 1990;Kitano and Chi 1989). These observations and their relationship to differences in the genotypic distribution of ALDH2 alleles among Asian groups warrant further investigation.
Future studies also should assess the interactions between different genes (e.g., between ALDH2 and ADH genotypes) in shaping a person's drinking behavior. Fi nally, genetic factors, such as ALDH2 and their functions, have been largely unstudied in Asian women, and it is thus unclear to what extent biological or cultural differ ences explain gender differences in alcohol use and alcoholism prevalence among Asians. Consequently, more studies that include Asian women are needed.
Analyses of the relationships between alcoholrelated genes, such as ALDH2, and behavior can provide insight into the deter minants of alcoholuse patterns of people at various levels of genetic risk for alco holism. These studies will contribute sub stantially to our knowledge of the causes of alcoholism and may help to improve alco holism prevention and treatment. A better understanding of how the interactions of other biological and cultural factors with 7 Acculturation refers to the degree to which immigrants adopt the values, attitudes, and behavior of the culture they enter. the ALDH2 genotype help determine the risk of Asians for alcoholism also may have important implications for evaluating the predisposition to alcoholism of other, non Asian populations. ■